1. Field of the Invention
This invention relates to impact printers, and more specifically, to maintaining the ink content on the print ribbon of such printers.
2. Related Art
The prior art of impact printing in line matrix printers is accomplished when hammers are released from retention. This causes their hammer tips to strike against an inked ribbon as it traverses between the hammers and the print media. The print media is backed-up on the other side by a hard platen, so that the impact from the hammer tip leaves ink dots on the print media. The print media can be paper, labels, multi-layer forms, including plastic and combinations of plastic and paper.
The inked print ribbon traverses at an angle between a single or dual row of hammers and the media. Each hammer strikes against the print ribbon in a dedicated zone running the length of the ribbon. The ribbon width and angle of inclination are such that the edges of the inked print ribbon are generally not struck by the hammers. This provides a boundary of tolerance to accommodate dimensional variations.
In certain line matrix printers, the ribbon reciprocates between two spools. The ribbon reverses direction when either of the spools becomes empty of ribbon. In others, the ribbon is continuous and circulates in a loop from a cartridge across the print hammers.
In dual-row hammer line matrix printers as opposed to single row hammer line matrix printers, the arrangement is slightly more complicated. In such cases the two rows of hammers simultaneously print adjacent rows of print. This effectively doubles the throughput of the printer.
To accommodate the two rows of hammers, an inked print ribbon traverses at a slightly shallower incline across the hammers than in a single-row printer. The result is that the middle area of the ribbon is struck twice during each pass of the ribbon, while the outer boundaries are only struck once. This has adverse print quality effects. The defects in print quality when ink is depleted from a ribbon whether it be a dual-row hammerbank or a single-row hammerbank can become quite apparent.
When the ink supply in the ribbon gradually decreases, it causes undesirable effects. Firstly, the density, or darkness of the printed dots decreases continuously as ink is consumed. Thus a page printed near the end of the ribbon life is much lighter than a page printed from a fresh ribbon.
Secondly, pre-inked ribbon becomes damaged as ink is consumed. This is because the ink, which lubricates the ribbon fibers, is depleted. Damaged ribbon can result in print failure at the edges of the media, as well as certain kinds of mechanical failure such as paper jams and hammer print tip clogging.
When hammer strikes are toward the center of the ribbon the unused borders of the ribbon retain a disproportionately large quantity of ink. This larger quantity of ink slowly diffuses toward the center of the ribbon. This produces darker dots on the edges of the printed page than are produced elsewhere. This effect in the art is referred to as the diffusion effect.
The fact that the middle portion of the ribbon in dual-row printers is struck twice, means that the ink is depleted more rapidly from that portion than from the edges. After a relatively small amount of printing, a light and dark pattern appears in adjacent lines of print. One of the two printed lines, for instance that which is printed by the upper row of hammers, will be darker on the right side than on the left. The next line printed by the lower row of hammers will be darker on the left and lighter on the right. In the art this is referred to as banding.
Uneven printing demands in various forms and orientations present substantial depletion of ink on a ribbon in uneven patterns. For example by printing only on the left side of the media, or by printing heavy graphics in one specific area of a page, repeatedly for many pages, can cause the print density to vary across the width of subsequent pages. This defect in the printing art is referred to as the column effect.
Another consideration is the inherent flexibility of impact printers. Such printers handle a wide range of print media. This results in a concomitant range of ink absorption rates. Consequently, ink depletion varies with print media, and location of printing on the media.
To overcome the foregoing problems, the art has developed re-inking devices. However, these re-inking devices typically only apply ink uniformly over the entire ink ribbon, while other re-inking devices generally re-apply ink to the ribbon without sensing areas in need of ink. These types of re-inking devices may not produce uniform printing when specific portions of the ribbon are used heavily or lightly in relation to the other portions of the ink ribbon.
Accordingly, it is desirable to apply ink to ribbons of impact printers that overcomes the deficiencies discussed above.